Computer display monitors provide a convenient interactive capability which virtually all interactive computer systems rely upon. One of the most common types of computer display monitors is that utilizing a cathode ray tube display system. In such monitors, a cathode ray tube is provided which directs one or more intensity modulated electron beams at a phosphor display screen. In systems utilizing a raster scan, the electron beams are scanned at different frequencies in the horizontal and vertical display directions while the electron beams are intensity modulated by the video information to produce the desired video display. Because it is generally desirable to provide relatively flat display screens housed with compact monitor housings, the geometric relationships between the cathode ray tube electron source or sources and the display screen gives rise to a variety of geometric distortions of the scanned raster. In addition, other sources of distortion such as scanning system nonlinearity exacerbate these geometric distortions. In virtually all computer monitors, the scan process is adjusted or corrected to accommodate these problems in a process generally known as "geometric correction". Basically, this process involves utilizing the horizontal and vertical scan rate signals available within the scanning and synchronizing system to generate corrective signals. Since the use of such correction systems relies upon the appropriate relationship between horizontal and vertical scan signals, their synchronization and timing, and the corresponding predictable area of the raster scan display, accurate geometric correction requires reliable synchronizing signals.
The process of geometric correction is further complicated by the tendency of most computer display monitors to operate at a plurality of scan frequencies often extending over a substantial frequency range.
Thus, there arises a need in the art for reliable scan synchronizing systems to supply the necessary signals for accurate geometric correction of the display appropriate for use in the demanding environment of multifrequency display monitors.
Accordingly, it is a general object of the present invention to provide an improved geometric correction system. It is a more particular object of the present invention to provide a reliable source of accurately synchronized scan related signals for use in forming geometric correction signals suitable for use in multifrequency monitors.